Climate change is measurably slowing Earth’s rotation, researchers find

Earth’s 24-hour day is getting imperceptibly longer as the planet’s rotation slows—a shift measured in thousandths of a second per century. A new geophysics study argues that, over the last two decades, human-driven climate change has become a detectable contributor to that slowdown, and that the climate-related signal appears unmatched in at least the last 3.6 million years.

In research published this month in the Journal of Geophysical Research: Solid Earth, a team led by Mostafa Kiani Shahvandi of the University of Vienna reports that from 2000 to 2020, climate-related processes lengthened the day at an effective rate of about 1.33 milliseconds per century. The main driver, the authors say, is the accelerating melt of polar ice sheets and glaciers, which raises sea level and shifts mass toward the equator—slightly reducing Earth’s spin rate.

“This rapid increase in day length implies that the rate of modern climate change has been unprecedented at least since the late Pliocene, 3.6 million years ago,” co-author Benedikt Soja, a professor of space geodesy at ETH Zurich, said in a statement. “The current rapid rise in day length can thus be attributed primarily to human influences.”

A figure skater in slow motion

The physics is familiar: when a spinning body moves mass farther from its axis of rotation, it slows down, much like a figure skater who spins more slowly after extending their arms.

“In our earlier work, we showed that the accelerated melting of polar ice sheets and mountain glaciers in the 21st century is raising sea levels, which slows Earth’s rotation and therefore lengthens the day—similar to a figure skater who spins more slowly once they stretch their arms,” Shahvandi said.

As ice in Greenland, Antarctica and mountain ranges melts, water drains into the oceans and redistributes mass across the surface. More of that mass ends up farther from the planet’s spin axis, increasing Earth’s moment of inertia. With angular momentum conserved, Earth’s rotation rate decreases slightly and the length of a day increases.

The effect is extremely small. At the climate-driven rate identified for 2000 to 2020, it would take tens of thousands of years to add a full extra second to the length of the day—far beyond what anyone would notice on everyday clocks.

Reading the length of day from ancient seas

To compare today’s changes with the past, the researchers needed a longer record than direct observations can provide. Ultra-precise space-age measurements of Earth’s rotation span only recent decades, so the team turned to paleoclimate evidence preserved in ocean sediments.

The study relies on benthic foraminifera—single-celled organisms whose shells accumulate on the seafloor. Their shell chemistry reflects seawater conditions, enabling scientists to infer historical changes in global sea level and ice-sheet volume.

“From the chemical composition of the foraminifera fossils, we can infer sea-level fluctuations and then mathematically derive the corresponding changes in day length,” Shahvandi said.

The team fed reconstructed sea-level histories into a “physics-informed diffusion model,” described as a probabilistic deep-learning approach that incorporates the physics of sea-level change and Earth rotation while accommodating large uncertainties in ancient data.

Over the last 3.6 million years—a period spanning multiple ice ages and warmer intervals—the authors report that climate-driven changes in day length rose and fell as ice sheets advanced and retreated. Only one episode about 2 million years ago produced a climate-related lengthening that approaches recent decades, and the paper concludes that even that event appears weaker than the modern trend.

The researchers emphasize they are isolating the portion of day-length change tied to sea level and climate. Other influences—particularly lunar tidal friction and interactions between Earth’s mantle and liquid outer core—also affect rotation on different timescales.

Space-age clocks and satellites confirm the modern signal

For the modern era, the climate-linked increase in day length is constrained using some of the most precise geodetic techniques available.

Very Long Baseline Interferometry (VLBI) uses networks of radio telescopes to observe distant quasars; tiny differences in signal arrival times reveal Earth’s orientation in space. Additional constraints come from satellite laser ranging and global navigation satellite systems such as GPS and Europe’s Galileo. The International Earth Rotation and Reference Systems Service synthesizes these observations into official estimates of Earth orientation parameters, including length of day.

The new paper connects that satellite-era framework to deep-time reconstructions, enabling a direct comparison between today’s climate signal and those inferred during past glacial cycles.

Why milliseconds matter

A change of 1.33 milliseconds per century is negligible for daily life, but it matters for systems that depend on precise timing and positioning.

“By the end of the 21st century, climate change is expected to affect day length even more strongly than the Moon,” Soja said. “Even though the changes are only milliseconds, they can cause problems in many areas, for example in precise space navigation, which requires accurate information on Earth’s rotation.”

Deep-space navigation, satellite positioning, telecommunications networks and other timing-sensitive infrastructure rely on accurate models of Earth’s rotation.

Since 1972, Coordinated Universal Time (UTC) has been kept close to Earth’s rotational time by the occasional addition of a leap second. All 27 leap seconds added so far have been positive, reflecting a long-term slowing driven largely by tides. In recent decades, however, flows in Earth’s core have slightly sped up rotation, raising the possibility of a first-ever negative leap second.

A 2024 Nature study concluded such a negative leap second could be needed around 2029, and reported that climate-related ice melt has already delayed that timeline by slowing Earth back down. Meanwhile, the General Conference on Weights and Measures decided in 2022 to phase out leap seconds by 2035, with technical bodies now working through implementation details.

A subtle but far-reaching signal

Scientists caution that reconstructing climate-driven changes in day length over millions of years involves multiple layers of inference, each carrying uncertainty. Translating fossil shell chemistry to sea level and then to rotational effects is complex, and the approach will require further testing and refinement.

The study does not claim climate change dominates all variations in Earth’s rotation. Over long periods, tidal friction remains the primary driver of a longer day, while core, atmospheric and ocean dynamics create shorter-term fluctuations.

Still, the authors conclude that when isolating the component tied specifically to sea-level and climate-driven mass redistribution, the pace since 2000 has no match in the last 3.6 million years of their record—offering another, independent measure of how rapidly human activity is reshaping the planet.